1. Field of the Invention
This invention relates to a new sheet-like chemical cell and a manufacturing method thereof, a fuel cell and a manufacturing method thereof.
2. Related Art
Thanks to the progress of recent electronic technologies, electronic devices such as mobile telephone sets, book-type personal computers, audiovisual equipment, and mobile information terminal equipment have been downsized and become rapidly widespread as portable electronic devices. Conventionally, such portable electronic devices are systems driven by secondary batteries. The secondary batteries have been developing in the order of sealed lead-acid battery, Ni—Cd battery, nickel metal-hydride battery, and lithium-ion battery, that is, thanks to miniaturization, weight saving, and high energy density technologies. To prolong service lives of these secondary batteries by one charging, various improvements have been made such as development of battery activating materials and high-capacity storage structure to increase their energy densities.
However, the secondary batteries require recharging after a preset quantity of power is used and consequently, they require charging devices and comparatively long charging periods. The portable electronic devices must solve these problems. The coming portable electronic devices will require power supplies of higher energy densities and longer running periods per recharging as they must process greater amounts of information at higher processing rates. In other words, the world is expecting smaller power generators (or micro generators) that need no recharging.
As one of such power generators that meet the above requirements, a fuel cell power supply has been discussed. A fuel cell converts chemical energy of fuel directly into electric energy (electrochemically) and requires no engine section although a conventional engine generator uses an internal combustion engine. Accordingly, the fuel cell has a high possibility as a compact power generating device. Further, the fuel cell need not stop the operation of the portable electronic device temporarily for recharging as frequently shown in the use of secondary batteries as it keeps on generating power as long as fuel is supplied to the fuel cell.
A polymer electrolyte fuel cell (PEFC) is well known as a high current density cell to meet the above requirements. The PEFC comprises an electrolyte membrane made of perfluorocarbon sulfonate resin, an anode, and a cathode and generates electricity by oxidizing the hydrogen gas at the anode and reducing the oxygen gas at the cathode. To make this PEFC smaller, for example, Japanese Patent Laid-open09-223507 discloses a small PEFC power generating device comprising a cylindrical cell assembly which has an anode and a cathode respectively on the inner and outer surfaces of a hollow fiber electrolyte wherein hydrogen gas and air are supplied respectively to the inside and the outside of the cylinder.
However, as a hydrogen gas is used as the anode gas and its volumetric energy density is low, the fuel tank must be greater when the small PEFC power generating device is applied as a power source to a portable electronic device. Further, this power generating system requires auxiliary machines to supply fuel gas and oxidizing gas (e.g. air) to the electrolyte membrane in order to maintain the performance of the cell. Finally, this makes the power generating system complicated and it cannot be said that the cell is small enough.
To increase the volumetric energy density of fuel, it is effective to use a liquid fuel and to simplify the cell structure (e.g. by eliminating the auxiliary machines to supply fuel and oxidizing agent to the cell). For this purpose, some proposals have been made. Japanese Patent Laid-open 2000-26885 and Japanese patent Laid-open 2000-268836, which are the latest examples propose a direct methanol fuel cell (DMFC) which uses methanol and water as the fuel.
The DMFC comprises a liquid fuel container, a material, which can supply liquid fuel by the capillary action on the outer wall of the container, an anode in contact with the material, a polymer electrolyte membrane, and a cathode, the members being disposed in that order. As oxygen is supplied by dispersion to the outer surface of the cathode, which is in contact with the outside air, this type of power generation device does not require any auxiliary machine to supply the fuel and the oxidizing agent. This simplifies the fuel cell system. The DMFC has a feature that the unit cells can be connected in series feature that the unit cells can be connected in series simply by electrical connections and require no cell connecting part such as a separator. However, when the DMFC is connected to a load, the output voltage of a unit cell is very low (0.3 to 0.4V). Therefore, to generate an output voltage available to portable electronic devices, you must connect unit cells in series using a plurality of fuel tanks for the cells.